Intrinsically safe system including electrical barrier with external connectors

ABSTRACT

A system having a nonhazardous area and a hazardous area electrically intercommunicate via an electrical barrier. The barrier fails safe, limits both voltage and current, and consists essentially of a monolithic, nonrepairable, ground-bus-mounted circuit having a fuse and three resistors in series, and a pair of zener diodes which break down with excessive voltages at intermediate points of the series resistance.

United States Patent [72] inventor Bennett l-lallenbeck 2,963,57712/1960 Errichiello 317/101 Tustin, Calif. 3,106,665 10/1963 Byles317/100 [21] App] No. 829,491 3,353,068 11/1967 Turk 317/99 [22] FiledJune 2, 1969 [45] Patented Oct. 19, 1971 OTHER REFERENCES 8" yCorporation Presented to Parliament by the Minister of Labour, Annual RcheslenN-Y- Report of H.M. Chief Inspector of Factories, 1966, pp. 79-

81, Her Majesty s Stationery Office, London, England 541 INTRINSICALLYSAFE SYSTEM INCLUDING Primary Examiner-Lewis Myers ELECTRICAL BARRIERWITH EXTERNAL Assistant Examiner-Gerald P. Tolin CONNECTORSAttorneys-Peter J. Young, Jr. and Joseph C. MacKenzie 7 Claims, 8Drawing Figs.

[52] US. Cl 317/99, 317/100, 317/101 R [5]] Ill. ABSTRACT: A tem ha inga nonhazardous area and a [50] Fleld of Search hazardous area l t i ll it r mmunicate via an electrical 317/99-101; 174/526; 307/ 2, 93 barrier.The barrier fails safe, limits both voltage and current, 56 R f d andconsists essentially of a monolithic, nonrepairable, l e "wees noground-bus-mounted circuit having a fuse and three resistors UNITEDSTATES PATENTS in series, and a ,pair of zener diodes which break downwith ex- 2,737,579 3/1956 Wehrlin 317/101 cessive voltages atintermediate points of the series resistance.

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FIG. 7

BARRIERS BENNETT L. HALLENBECK S U m n HR A H O INVENTOR.

FIG. 8

ATTORNEY INTRINSICALLY SAFE SYSTEM INCLUDING ELECTRICAL BARRIER WITIIEXTERNAL CONNECTORS The background of the invention is theconcept ofintrinsic safety. According to this concept, electrical apparatus,potentially dangerous to life, limb and/or property, is consideredintrinsically safe, provided that under no reasonably foreseeablecircumstances whatsoever can-such apparatus produce electrical sparks,electrical overheating,,excessive levels of voltage or current, humanhealth or safety hazards, and the like. While intrinsic safety impliesin some sense protection of the electrical apparatus as such, it isbasically a matter of assuring that if electrical apparatus damage orfailure does occur,,for' whatever reason, nothing worse will result. Forinstance, if the apparatus is mechanically damaged, and intrinsic safetyprevails no sparks or overheating will occur such as might ignite aninflammable atmosphere contacting the apparatus.

The prior art teaches employing fuses, zener diodes, circuit breakers,current-limiting resistors, and soforth, to provide intrinsic safety,safeguard apparatus, and the like. The various combinations andconfigurations of such elements, as have "been proposed heretofore, aretoo numerous to attempt to describe at length. A simple example,however, would be a series resistor and a zener diode. Theresistorlimitscurrent and the diode voltage appliedto some electrical device. Ifthevoltage and currentlimitsare such thatthe device can in nowaywhatsoever, and under no circumstance,produce ahazardous effect fromelectrical energy supplied'to it via the resistor then the device isintrinsically safe. In effect, oneis using the diode and resistor as abarrier preventing electrical energy from being applied to the device atdangerous or potentially dangerous levels.

In a system according to the present invention, electrical energy isconducted to a hazardous area via a fused resistor and diode circuitcontained in an insulative'envelope along with a metal heat sink, theheat sink providing diodesupport, the return ,path for the electricalcurrent and the means for mounting the barrier on, and electricallyconnecting it to, a ground bus external ,to the envelope. The envelopeis solidly filled with epoxy resin orlike potting material. The envelopeandits contents therefore form a rigid monolithicblock that issubstantially unrepairable, and provides a barrier that preventselectrical energy. above ,a given level from passing therethrough.

Preferably, the resistor and diode circuit has several stages. Thus, afirst zener diode diverts energy if the voltage across fuse and resistorbecomes excessive. If the excess lasts a fraction of an instant the fusewill blow if the current is excessive. Without the diode, excess currentcould persist long enough to be dangerous, before the fuse blew.Further, a second resistor is connected to the first resistor to alsocarry the energy current, and a second zener diode diverts the energy ifthe voltage across the second resistor becomes excessive. Preferably,the diodes are chosen so that the second actually fires before thefirst. A third resistor, finally is connected to the second resistor andalso carries the 'energy and limits this energy to a safe value.

In practice, it is found that this sort of electrical barrier permitsfreedom of choice and interconnections of all instruments and devices inthe nonhazardous or safe area. For example, a barrier according to theinvention can safeguard against a fault as high as 1,500 volts. Thetypical nonhazardous or safe area, however, is a control room complex inwhich rarely are encountered faults higher than 250 volts (R.M.S. orDC). On the other hand, equipment in the hazardous area is in generalintrinsically safe provided the barrier is operative to limit thecurrents and voltages applied to the equipment in the hazardous area.Preferably, the barrier according to the invention provides theelectrical interface between the safe and hazardous areas. The barrierof the present invention is practically foolproof, in contrast toexplosionproof housings and like structural expedients, since it cannotbe repaired and is more or less maintenance free.

Briefly described, FIG. I is an electrical diagram of the circuitry perse of a novel barrier according to the invention.

FIGS. 2 and 3 are plan and elevation views, partly in section, and IFIG. 4 is a bottom view of the actual barrier structure.

-FIGS.-5 and 6 correspond to FIGS. 3and 4 but show additional externaldetail, a portion of which is further illustrated in FIG. 7.

FIG. is a schematic viewof the invention in a typical use. FIG.-l showsthe basic barrier circuit, including specific circuit values of aworking embodiment. The diagram is entirely self-explanatory from thispoint of view except for the fmatched pair" of diodes Z and'Z,.Matching, in this case,

means that the diodes are chosen so that the voltage required .the sameground potential. As shown, the system is a negative ground system but,with reversal of the diodes,would-be a positiveground-system, amerematter of choice,.for present purposesExcept for the-indicated partsvalues, and the fuse F beingdistinctfromresistor R the circuit of FIG. lis old in the intrinsic safety barrier art.

In use, the electrical energysupplied bysource S normally will not causethecurrent throughfuse F to exceed its rated value, nor the voltage atthe cathodesof'the zener diodes to exceed diode-firing voltage.

.The instrumentality including load L is in the hazardous area, andnormal operation of the source and instrumentality is that theelectrical energy at terminals '3 and 4 is'below a level to which theinstrumentality can respond by creating a spark of like dangerouseffect, supposing'the hazardous area to be one with an inflammableatmosphere, for example. Under these conditions, the instrumentalityselectrical elements can be exposed to the inflammable atmosphere withabsolute certainty that ignition will not occur by reason'of abnormaloperation of the apparatus.

In a typical case, in normal operation,the systemresistance willbethe'source resistance, the same I50 ohms ofthe barrier circuit, andthe load resistance in series, and load voltage and'currentwillremainwithin safe limits.

However, it is-essential to assure the integrity of'the barrier in orderto obtain the advantages of its electrical properties (more particularlydescribed infra). In the present invention, the barrier is provided inmonolithic form for making the barrier circuit essentially unrepairableand inaccessible, in order to foil human intervention, namely attemptedmaintenance and/or repair. For instance, if the barrier circuit wereaccessibly housed in, say, a plastic box with removable cover, andappeared to be defective, the user might be tempted to replace adefective component that had failed safe because of its intrinsicfail-safe character, with an electrical equivalent, not intrinsicallyfail-safe, as for example, a molded carbon composition resistor, a typeof resistor which can too frequently fail short.

In the present invention, great precautions are taken to preventmaintenance or repair from being anything more than out-and-outreplacement of the barrier. in essence, the electrical circuit isembedded in insulating material contained in a box, itself of insulatingmaterial, to provide a monolithic assemblage that is easy to fabricate,and, once fabricated, cannot, as a practical matter, be disassembled.

Thus, FIG. 2 hereof shows that the safety circuit is housed in a box 5and that this box is solidly filled with a suitably insulating pottingmaterial 6 preferably an epoxy resin. FIG. 2 is a top view looking downinto the box through an open side that remains open except by virtue ofthe fill, the approximate level of which is indicated by the arrowheadof the reference numeral 6 in FIG. 2. It will be noted that the fill hasbeen broken out of the right-hand end of the box as shown in FIG. 2 andout of the left-hand end of the box as shown in FIG. 3. Taken together,the two figures show all the contents and interior of the box, which arealmost but not quite bilaterally symmetrical, the symmetry being spoiledby fuse F and the compartment therefor in the left side of box 5.

As will be seen from FIGS. 3 and 4, terminals 1 and 3 take the form ofconventional screw and washer connectors for electrical connection,respectively, to equipment in the safe 5 9 next the left-handcompartment 8 (FIG. 2), and has a pigtail 10 soldered to a stud 11forming part of left-hand block 7.

The other pigtail ll of fuse F is crimped to pigtail 12 of R by means ofa sleeve 13, and hangs more or less loosely in vertical position in themain compartment 14 of box 5. Pigtail 15 of R,, in turn is soldered withpigtail 16 of R to terminal 17 of 2,, which tenninal has a hole (such asshown at 18 in the case of 2,, FIG. 2) therein through which pigtails 15and 16 pass.

Diode Z is of the stud-mounted type wherein the mounting stud providesthe second electrode as shown, and is shrunk into a solid rectangularlyparapipedal brass block orheat sink S, and on the top of the sink S,rests R Diode Z is mounted like diode 2,. As will be seen from FIG. 2,the resistors are square in cross section so one face of R seats flushon the top face of block S. An insulating piece 16 of spaghetti coversthe pigtail 16 of R to obviate any possibility of the pigtail shortingto the sink S or the case of 2,.

As will be seen from FIG. 2, the right-hand end of box 5 is quite likethe left-hand end shown in FIG. 3, except that there is no fuse so thereis also no compartment 9. Thus, there is the pigtail 20 of R, solderedtogether with pigtail 20 of R both pigtails passing through the hole 18of terminal 22 of Z, and being soldered thereto, and having a piece ofspaghetti 23 on pigtail 20 to guard shorts to block S or the case of ZThe pigtail 24 of R is, of course, soldered to stud ll of the righthandle terminal block 7.

As FIG. 3 shows, terminals 2 and 4 of FIG. 1 take the form of nut andlockwasher pairs 25 which accommodate in the usual fashion ground orcommon leads 26 and 27, respectively. Lead 27 is the current return fromthe hazardous area to the barrier, and lead 26 is the current returnfrom the barrier to to the safe area. The bolts forming terminals 2 and3 pass through the casing 5 and thread into the heat sink to which theymake rigid conductive contact. Nuts 33 on terminals 2 and 3 pass freelythrough holes 34 in bottom 28.

As shown in FIGS. 3 and 5, the nuts 33 lock the terminals 2 and 3 to thesink S, and there is a gasket 34 under the sink S to seal between thebottom of sink S and the inner adjacent surface of bottom 28. Once theepoxy is hard, it supports the circuit and box 5 to the heat sink. Whenthe barrier is fixed to bus 0 G.B., the bus is clamped between nuts 33and the innermost nuts 25. This mechanically isolates the box 5 from thebus bar, to permit powerful clamping forces to be applied by nuts 25 tothe bus bar, thereby assuring good contacts to the bus bar withoutstressing the box 5 and its contents other than heat sink S. Bus G.B. isa flat bare, sturdy copper bar of any conventional longitudinal extentproviding for a good low-ohm ground or common, as well as substantialsupport for the barrier. Thus, as FIG. 3 shows by means of the brokenends of bus G.B. and the pair of holes 29 various numbers of barrierscan be clamped to the bus, like the barrier shown, but parallel theretoand spaced therefrom.

To assemble the barrier, one simply solders and otherwise assembles thecircuit elements, the heat sing S and the block 7 55 together as shown,and in the illustrated configuration. This assemblage is more or lessjust dropped into the box 5. Block 7 and sink S are temporarily clampedin place, and then a more or less liquid solution of epoxy resin isfilled into the remaining space. After the resin is hard, the temporaryclamping is removed.

When this is done, nothing of the circuit is exposed except terminals 1through 4, and when the resin hardens, one cannot get at the circuitwithout destroying the monolithic structure of connectors, circuitelements, box and fill. This is important in preventing tampering andill-advised or unauthorized attempts at repair or service of thebarrier. It cannot be stressed too much that the barrier should not berepaired except, perhaps, by what amounts to a rebuilding job performedby the manufacturer (through not feasible, due to the expense). It is tobe noted that this animus against repair or tampering is dietated notonly on engineering and manufacturing considerations, but also by therequirements of insurers of the installation affected by intrinsicsafety. In this connection, it is to be observed that the barrier asshown in acceptable to insurers as fulfilling the requirements forintrinsic safety under the condition of use described herein.

FIGS. 5, 6 and 7 illustrate the extemals of a typical commercialexample. In FIG. 5, a prominent label defines, so to speak, the area ofcompetence of the barrier and its criticality. In FIG. 6, a furtherlabel indicates its working voltage. The evident blank spaces on bothlabels provide for entering specific data as to specific instructionbook, approving insurer, catalog number, and the like, all of whichwould usually be printed on the lable (though omitted here).

The hazardous end of the barrier is at the terminal I, which, as can beseen from FIG. 4, is marked 1 adjacent terminal 1. Likewise, as toterminal 2, terminals 3 and 4. Terminals I and 5 3 are recessed into thebox 5. This provides for an end cover 30, having a hole 37 therein,intended to cover the right-hand end of the casing 5 with the hole 37registering with the recesscontaining terminal 3.

As will be seen from FIGS. 7 and 8, the casing and cover are slightlytrapezoidal, in order to assure that cover 30 only fits when the hole 37registers with terminal 3. Bottom 28 of box 5 has two ridges 31 and 35therein. Ridge 31 projects far enough to prevent cover 30 from beingforced so far on the box 5 as to crack the plastic cover 30. Ridge 35 isenough slighter to bring about force fit of cover 30 to casing 5, butnot so much as to crack cover 30. Preferably a slight ridge (not shown)is located in the inside of cover 30 that has to be forced past ridge 35to provide a positive lock against coming off the casing 5.

As shown in FIG. 6, a suitable label is applied to cover 30 to warn theuser not to remove the cover without finding out whether it isappropriate to do so or not. The location of hole 37 is to avoid havinga bend of wire lead between cover and the end of casing 5. Further, asFIGS. 2, 3 and 5 show, cover 30 cannot be put on wrong due to theinterferring projections 32 of the dangerous" end of the barrier.

Looking at the barrier overall, the hazardous end is at terminal 1. Thatis, it is this terminal that is to be kept out of the hazardous area.The end at cover 30, however, is the safe end, the one which can belocated in the hazardous area. While terminals 2 and 4 are in effectneutral, any short from terminal 3 to ground could be hazardous, unlessthe location of the short is outside the hazardous area. Hence, thecover 30 and of course, the insulation which would be on the lead goingfrom terminal 3 into the hazardous area, protect the safe end of thebarrier from contact with the dangerous elements of the safeenvironment. In any event, the best location for the barrier is just atthe edge of the safe area, at a point where there is the leastlikelihood of some unsafe connection to terminal 3.

In operation, normally the barrier is merely a series resistance. Aslong as it thus operates, the user would have no concern with it beyond,say, inspecting to see that its external connections are in order, cap30 is on, and the like. Abnormally, however, the electrical energy attenninals 1 and 2 will exceed safety limits.

The barrier circuit is designed to maintain safety limits long enoughfor the fuse to blow. It is further designed to do this even withfailure of one or another of its components. The resistors R R and R arewire-wound resistors which can only fail-safe, that is byopen-circuiting. If a resistor burns out, source S is simplydisconnected from load L, automatically.

The diodes Z and Z are of a type that fail almost invariably by shortcircuiting. Thus, 1N2989B Zener diodes fail almost invariably by opencircuiting. Diodes of this specific type, and

other types also having this fail-safe" property are routinely availablefrom various manufacturers. As will be seen later, both diodes must failsimultaneously to defect the purpose of the circuit, which means thatbarrier failure due to diode failure has a probability of about 1 inmillions. I

The fuse F is the weakest link. Compared to the time it takes a diode tofire in response to voltage at a firing level, the fuse F (despite beingpreferably a so-called fast-blow type) takes a relatively long time toblow out. Further, the normal current rating of the fuse is by no meansa reliable index of how much current it will stand, without blowing out.However, the diode/resistor arrangement assures that the fuse blowsbefore the energy available at terminals 3 and 4 reaches a dangerouslevel.

Anyv dangerous level of electrical energy applied to terminals l and 2will increase the voltages across the diodes. The difference between thefiring voltages of the diodes assures that the first diode to fire willbe diode Z When 2,, fires, this immediately increases the level ofcurrent through fuse F. This increase is due to the fact that Z firing,provides a low resistance shunt to ground'across R and the loadresistance in series, each of which is large compared to the resistanceto ground via the firing diode. Ordinarily, fuse F will bow while Zregulates the voltage across it to its firing voltage and shunts currentto ground.

However, it is just possible that fuse F will not blow, and indeed willbe able to sustain enough current that Z becomes overdriven. If 2 isoverdriven, its regulating and shunting abilities saturate. Now, thevoltage across it can rise substantially above its rated voltage, andmore energy could be applied to the load, possibly at a dangerous level.

However, if fuse F does not blow and the voltage across Z begins torise, 2 fires before the voltage across 2 exceeds a safe value. Z firingdecreases the resistance seen by source A still further, causing afurther increase in current through fuse F. Furthermore, Z firing allowsZ to regulate normally, since 2 is shunting current to ground, and canindeed be overdriven, while Z continues to regulate in normal zenerfashion. Finally, as long as both zeners do not simultaneously fail byopen circuiting, the worst that can happen is that a diode fails bybecoming a short circuit, thereby dropping the voltage across terminals3 and 4 to substantially zero and burning out fuse F while divertingsubstantially all current from the load until the fuse blows.

It will be observed that the above sequence of events, if it occurred,would occur in much less time than it takes to describe it.

It will be seen, therefore, that it is impossible that the barriercircuit be in any but one or the other of two states, one being that ofa conductor having some ISO-ohms resistance between terminals 1 and 3,and the other being that of a conductor open circuited between terminals1 and 3.

Once in use, the barrier becomes in effectjust another connector, or,more precisely, just another fuse. However, it can be tested,nondestructively, to determine its state of operativeness relativelycompletely. Thus, if the barrier circuit is built according to FIG. 1,the resistance between terminals 1 and 3 would be measured. If theresistance from terminals 1 to 3 is other than 151.6 ohms :5 percent,plus 0.26 percent per C. referred to 25 C., the barrier should berejected as suspect.

Further test would be to drive the barrier circuit of FIG. 1 with acurrent source at 100 ma. DC with terminals 3 and 4 disconnected. Thatis to say, the voltage across terminals 1 and 2 would be sufficient tofire both diodes, without, however, blowing the fuse F, in this case thefollowing Fault" table applies:

Z and/or 2 shorted and/or reversed 0 to 0.7 typical .rrelevantMeasurements should be made, of course. with a voltmeter of theindicated accuracy, and having at least l-megohm input resistance.

When a diode fires, the load sees what appears to be a 30- volt batteryin series with ohms, during such time as the fuse manages to stay open.This characteristic allows the transmission line into the hazardous areato be calculated for the worst case. The worst case, of course, is whenthe transmission line impedance is equal to the barrier outputimpcdance. Hence, the amount of energy that can be delivered to the loadcan be decreased simply by suitable mismatch to the barrier. It isfurther to be noted that by increasing the sum of fuse resistance, R Rand R to 300 ohms, the circuit of FIG. 1 provides intrinsic safety forinstrumentation in a hydrogen atmosphere. Series resistance, however,has also to be chosen with regard to the load. The some ohms exemplifiesa value suitable for nominal 24-volt instrumentation in common usetoday. Diode leakage, incidentally, in the circuit of FIG. 1 hasinsignificant effect on accuracy of transmission.

With a barrier of the form shown in FIG. 1, a reliable estimate of theprobability of hazardous failure in a typical system is on the order ofone in a million, million, million. This order of reliability isliterally astronomical. As an illustrative analogy, one might expect anautomobile in good condition to go 40 miles without failure, based onthe reliability of typical automobiles, whereas a vehicle having areliability corresponding to that of the barrier according to theinvention could make several round trips from earth to sun; with lessexpectation of a single failure occurring during the whole series ofround trips.

The reliability of the barrier according to the invention has beenrecognized by industrial insurers, to the extent that the barrieraccording to the invention is approved by Class I, Division l, Groups Cand D use. Class I, for example, includes gaseous environments whereininflammable atmospheres including ethyl ether, ethylene, andcyclopropane can normally or occasionally provide the inflammable partof the atmosphere. For instance, surgical installations are likely to beClass I, Division I or II, Group C environments. The difference betweenDivisions 1 and II is that the former can be normally hazardous whereasthe latter is not ordinarily hazardous. Incidentally, the above ratingsare conservative. There are not to be taken as limiting. Theirimportance is that insurers and like safety-concerned entities haveofficially granted such ratings. As pointed out previously, the barriercan be modified to make hydrogen atmospheres safe, as so-called class Bhazard, through this at this time has not been sanctioned by authority.

FIG. 8 shows a typical installation wherein the safe area is a controlroom, or the like, containing a control cabinet 50, wherein variousequipment 51 is to be connected to entities 61 to 64 in the hazardousarea. A ground bus G.B. having four barriers thereon is shown at the topof the cabinet 50. At the right ends of the barriers, wires lead out tothe hazardous area through the room wall. At the left ends of thebarrier, wires go to equipment in the control cabinet. Bus B.G., ofcourse, has a good ground connection (not shown).

The safe area control cabinet 50 will normally contain variouselectrically operated process indicators, controllers, and powersupplies, AC mains outlets, electrical computers or output terminalsthereof, and the like. The hazardous area, on the other hand, willcontain various process equipment, and electrical measuring andcontrolling devices which are electrically coupled by the barriers tothe aforesaid electrical equipment. If the hazardous area containsinflammable material, the electrical entities can be safely exposed tothe inflammable material, provided that these entities can be preventedfrom producing heat or sparks capable of ignition. The barrier performsthis function and hence obviates such expedients as explosionproofhousing, and the like structural artifices by means of which it issometimes attempted to allow dangerous electrical phenomena to arisesafely in hazardous areas by structurally isolating them. Thesestructural artifices are cumbersome, and great care must be taken tokeep them in working order since failure is readily overlooked. Forinstance, malfitting of the cover of an explosionproof instrumenthousing, easily overlooked, can leave a flame-propagating path betweenelectrical equipment within the housing and an external flammableatmosphere. That is to say, structural explosionproofing is not"intrinsically safe." The instrument within the housing may be perfectlyharmless in normal operation, that is when its current and/or voltagelevels remain those it was designed for. The housing, however, cannotprevent these levels from being exceeded in consequence of mishaps, ormalfunctions external to the hazardous area.

The barrier according to the invention, on the other hand, allows nodangerous voltage or current levels to be introduced into the hazardousarea in the first place, hence, it is perfectly safe to allow theflammable atmosphere to contact the electrical equipment (connected toelectrical barriers according to the invention) in the hazardous area.

The barrier illustrated herein is substantially exactly in accordancewith a commercial example of the invention. The parts of the circuit areall routinely available from various manufacturers stocks, and inassembling the circuit, the only selective step is to choose the diodessuch that the zener voltage of diode Z exceeds that ofZ by about a volt.

Certain economies can be practiced with the illustrated structure, butwith results degrading to the intrinsic safety character. For instance,first sight suggests that it is really most uneconomical to discard abarrier because of the failure of fuse F (typical cost, 12 cents).Unfortunately, if users can get access to the fuse for replacement withthe wrong fuse, or even something that is not a fuse at all. This sortof thing is potentially very dangerous. From one point of view, thewhole point of the barrier is to make sure that a correctly chosen fuseflows in time enough to keep dangerous levels of electrical energy fromarising in the hazardous areas. Since the purpose of the barrier islikely to be to prevent blowing up thousands of dollars of equipment, orworse, both practice and theory demand that the barrier be out-and-outreplaced after it has done its job but once.

Nonetheless, certain modifications are possible. R for example, is aprecision resistor which together with 2 precisely define a DC source ofgiven properties, with respect to the load into which it works. Thisload will include transmission line and an instrument with knowncapacitive and inductive properties. Knowing the source propertiesprecisely allow one to determine the worse case, namely, that amount oftransmission line, etc., into which the source can transfer energy withoptimum efficiency. This allows source impedance and load impedance tobe mismatched, as by lengthening, or shortening transmission line,enough to prevent occurrence of the worst case should this worst casehappen to be borderline insofar as intrinsic safety is concerned. On theother hand, R is essentially also part of the load, and in some casescould be structurally such.

Again, R and F are essentially one resistance, but as a matter ofavailability it is usually more practical to have them as separableentities. Again, terminals 1 and 2 are driven by a battery or generatorof some sort, and of course Z cannot distinguish among generatorimpedance, fuse F and resistor R,, so in a rare case these threeentities could be lumped into one.

Zener diodes are not the only kinds of breakdown devices available, butare recommended due to ready availability of a wide spectrum ofproperties, manufacturers, etc. of zener diodes. For solid-state,low-voltage systems, which are contemplated here, zener diodes areideal.

Having set forth my invention in accordance with USC 35 1 12, I claim:

1. An intrinsic safety barrier comprising a box, a first externallyaccessible electrical connector means in the external surface of saidbox, and a second externally accessible electrical connector means inthe external surface of said box, the locations of said connector meansbeing separated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; andpotting means nonrepairably enveloping and potting said circuitry insaid box said box having an open face and compartments therein openingtoward said open face, components of said circuitry being in saidcompartments, said potting means filling said box substantially to saidopen face.

2. An intrinsic safety barrier comprising a box, a first externallyaccessible electrical connector means in the external surface of saidbox, and a second externally accessible electrical connector means inthe external surface of said box, the locations of said connector meansbeing separated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; pottingmeans nonrepairably enveloping and potting said circuitry in said boxand a supporting block in said box, said block mounting components ofsaid circuit and being secured to said box by said potting means.

3. An electrical barrier system comprising a first resistor, a secondresistor, a third resistor, a first diode, and a second diode; saidfirst resistor having one end connected to one end of said secondresistor, and to one electrode of said first diode, said third resistorhaving one end connected to the other end of second resistor and to oneelectrode of said second diode,

each said one electrode being like the other said one electrode, saidsystem further comprising a source of electrical energy and a loaddevice, a first DC conductor, a second DC conductor and a third DCconductor, said source having a first terminal connected to said firstDC conductor, said load device having a first terminal connected to saidfirst DC conductor, and the other electrodes of said diodes beingconnected to said first DC conductor, said source having a secondterminal connected to the other end of said first resistor via saidsecond DC conductor, said load device having a second terminal connectedto the other end of said third resistor via said third DC conductor;said source having its internal impedance effectively connected betweenits said terminals, and said load device having its internal impedanceconnected between its said terminals, and the relative polarity of saidsource and said diodes being such that said diodes do not conduct exceptwith the voltage thereacross due to said source exceeding theirbreakdown voltages; said second conductor having as part thereof a fusethrough which all the current between said first terminals flows; saidbarrier system including a box containing said resistors and saiddiodes, and potting means nonrepairably enveloping and potting saidresistors and diodes in said box; said box having a first portion ofsaid third conductor projecting thereinto for connection to said otherend of said third resistor, said first portion being an electricalconnector recessed into the external surface of said box; said conductorhaving a second portion fixed to said connector passing therefrom in theform of an insulated wire to the remainder of said conductor; said boxhaving a cap fitted thereon, said cap having an aperture therein, andfastening means fastening said cap to said box with said apertureregistering with said electrical connector and having said wire passingtherethrough.

4. An intrinsic safety barrier comprising a box, a first externallyaccessible electrical connector means in the external surface of saidbox, and a second externally accessible electrical connector means inthe external surface of said box, the locations of said connector meansbeing separated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; pottingmeans'nonrepairably enveloping and potting said circuitry in said box,and a cap fitted to said box and covering said one said electricalconnector means and having an aperture therein registering with said onesaid connector means.

5. An intrinsic safety barrier comprising a box, a first externallyaccessible electrical connector means in the external surface of saidbox, and a second externally accessible electrical connector means inthe external surface of said box, the locations of said connector meansbeing separated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; pottingmeans nonrepairably enveloping and potting said circuitry in said box,and a supporting block, said block mounting components of said circuitand being secured to said box by said potting means, said block havingmounting means fixed thereto for mounting said box, said mounting meansbeing located externally of said box,

6. An intrinsic safety barrier comprising a box, a first externallyaccessible electrical connector means in the external surface of saidbox, and a second externally accessible electrical connector means inthe external surface of said box, the locations of said connector meansbeing separated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; pottingmeans nonrepairably enveloping and potting said circuitry in said box, asupporting block, said block mounting components of said circuit andbeing secured to said box by said potting means, and third externallyaccessible electrical connector means in the external surface of saidbox and separated from the two previously said connector means byexternal surface of said box, said third connector means beingconductively fixed to said block, said block being made of conductivematerial conductively connected to said circuit.

7. The barrier of claim 6, wherein said third electrical connector meansprovides mounting means externally of said box for mounting said box.

1. An intrinsic safety barrier comprising a box, a first externallyaccessible electrical connector means in the external surface of saidbox, and a second externally accessible electrical connector means inthe external surface of said box, the locations of said connector meansbeing separated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; andpotting means nonrepairably enveloping and potting said circuitry insaid box said box having an open face and compartments therein openingtoward said open face, components of said circuitry being in saidcompartments, said potting means filling said box substantially to saidopen face.
 2. An intrinsic safety barrier comprising a box, a firstexternally accessible electrical connector means in the external surfaceof said box, and a second externally accessible electrical connectormeans in the external surface of said box, the locations of saidconnector means being separated one from the other by external surfaceof said box; current-limiting and voltage-limiting circuitry containedwithin said box and interconnecting said connector means with eachother; potting means nonrepairably enveloping and potting said circuitryin said box and a supporting block in said box, said block mountingcomponents of said circuit and being secured to said box by said pottingmeans.
 3. An electrical barrier system comprising a first resistor, asecond resistor, a third resistor, a first diode, and a second diode;said first resistor having one end connected to one end of said secondresistor, and to one electrode of said first diode, said third resistorhaving one end connected to the other end of second resistor and to oneelectrode of said second diode, each said one electrode being like theother said one electrode, said system further comprising a source ofelectrical energy and a load device, a first DC conductor, a second DCconductor and a third DC conductor, said source having a first terminalconnected to said first DC conductor, said load device having a firstterminal connected to said first DC conductor, and the other electrodesof said diodes being connected to said first DC conductor, said sourcehaving a second terminal connected to the other end of said firstresistor via said second DC conductor, said load device having a secondterminal connected to the other end of said third resistor via saidthird DC conductor; said source having its internal impedanceeffectively connected between its said terminals, and said load devicehaving its internal impedance connected between its said terminals, andthe relative polarity of said source and said diodes being such thatsaid diodes do not conduct except with the voltage thereacross due tosaid source exceeding their breakdown voltages; said second conductorhaving as part thereof a fuse through which all the current between saidfirst terminals flows; said barrier system including a box containingsaid resistors and said diodes, and potting means nonrepairablyenveloping and potting said resistors and diodes in said box; said boxhaving a first portion of said third conductor projecting thereinto forconnection to said other end of said third resistor, said first portionbeing an electrical connector recessed into the external surface of saidbox; said conductor having a second portion fixed to said connectorpassing therefrom in the form of an insulated wire to the remainder ofsaid conductor; said box having a cap fitted thereon, said cap having anaperture therein, and fastening means fastening said cap to said boxwith said aperture registering with said electrical connector and havingsaid wire passing therethrough.
 4. An intrinsic safety barriercomprising a box, a first externally accessible electrical connectormeans in the external surface of said box, and a second externallyaccessible electrical connector means in the external surface of saidbox, the locations of said connector means being separated one from theother by external surface of said box; current-limiting andvoltage-limiting circuitry contained within said box and interconnectingsaid connector means with each other; potting means nonrepairablyenveloping and potting said circuitry in said box, and a cap fitted tosaid box and covering said one said electrical connector means andhaving an aperture therein registering with said one said connectormeans.
 5. An intrinsic safety barrier comprising a box, a firstexternally accessible electrical connector means in the external surfaceof said box, and a second externally accessible electrical connectormeans in the external surface of said box, the locations of saidconnector means being separated one from the other by external surfaceof said box; current-limiting and voltage-limiting circuitry containedwithin said box and interconnecting said connector means with eachother; potting means nonrepairably enveloping and potting said circuitryin said box, and a supporting block, said block mounting components ofsaid circuit and being secured to said box by said potting means, saidblock having mounting means fixed thereto for mounting said box, saidmounting means being located externally of said box.
 6. An intrinsicsafety barrier comprising a box, a first externally accessibleelectrical connector means in the external surface of said box, and asecond externally accessible electrical connector means in the externalsurface of said box, the locations of said connector means beingseparated one from the other by external surface of said box;current-limiting and voltage-limiting circuitry contained within saidbox and interconnecting said connector means with each other; pottingmeans nonrepairably enveloping and potting said circuitry in said box, asupporting block, said block mounting components of said circuit andbeing secured to said box by said potting means, and third externallyaccessible electrical connector means in the external surface of saidbox and separated from the two previously said connector means byexternal surface of said box, said third connector means beingconductively fixed to said block, said block being made of conductivematerial conductively connected to said circuit.
 7. The barrier of claim6, wherein said third electrical connector means provides mounting meansexternally of said box for mounting said box.